A jackup rig is an offshore oil and gas exploration drilling structure or a work-over platform being used in shallow water, typically in water with depths up to 500 feet. The jackup rig usually comprises a floatable hull with a deck or working platform, and three or four legs, where the legs provide support for the floatable hull when the floatable hull is in elevated conditions. After the jackup rig arriving on location, the legs are lowered until they touch the underneath seabed and rest on the soil on the seabed; then the hull may be jacked up using a jacking system to raise the working platform above the water, making the jackup rig safe to be operated in open water situations where water movement is experienced.
The legs of a jackup rig are commonly tubular columns or trusses. One truss leg comprises vertical chords connected with cross braces that are normally diagonally disposed. When located in an arctic area, the leg needs to be able to accommodate large horizontal loads from incoming ice. An enclosed jackup leg such as a tubular column can achieve high strength, but its large, enclosed profile causes large loads from waves that can occur in summer when ice is not present. A traditional jackup truss leg is transparent to waves and so can withstand large waves by avoiding large forces; but the slender braces connecting the main chords are not designed to carry concentrated horizontal loads from sea ice and have limited capacity of withstanding these forces.